Modern motor vehicles have internal combustion engines with direct fuel injection in which the fuel is injected under high pressure directly into the combustion chamber, or combustion chambers in the case of multi-cylinder internal combustion engines. Such direct fuel injection requires a fuel supply device which makes available pressurized fuel in every operating situation. Significant elements of this fuel supply device are the high-pressure pump, which feeds the fuel at the necessary pressure level, and a pressure accumulator (rail) in which the fuel is stored under high pressure and from which the injection valves are supplied with fuel. In addition to the storage of pressure, the rail also serves to smooth pressure pulsations, for which a sufficient storage volume is necessary.
Such pressure accumulators are used, in particular, in high-pressure accumulator injection systems which are combined under the term “common rail” and which permit the injection pressure to be kept independent of the rotational speed of the internal combustion engine and of the injection quantity, and, furthermore, make it possible to raise the injection pressure to, for example, approximately 2000 bar. In these common rail injection systems, fuel from a fuel tank is fed by a high-pressure pump into the pressure accumulator, via which the fuel is present at injection valves which are each arranged in the cylinder heads of the internal combustion engine. The opening and closing of the injection valves generally takes place by means of electrically open-loop or closed-loop controlled actuators.
Endeavors by automobile manufacturers to reduce further fuel consumption and the emissions of motor vehicles have involved the development of new technologies such as, for example, the automatic stop/start function by means of which the internal combustion engine can be deactivated automatically independently of the intervention by a motor vehicle driver and can also be started automatically again without activating the ignition key or the starter knob, for example by the accelerator pedal or clutch pedal being touched by the driver's foot. The deactivation of the internal combustion engine takes place here, in particular, in relatively long idling phases in which the drive force of the internal combustion engine is not required. In this way it is possible to achieve considerable savings in terms of fuel consumption, particularly in town centre traffic with many stops at traffic lights.
Stop/start devices for internal combustion engines of motor vehicles are known, for example, from DE 10 2008 020 184 A1 and DE 10 2008 020 185 A1.
However, when such automatic stop/start functions are used, difficulties can occur in the fuel supply when the internal combustion engine is restarted. It is important here that the time between the activation, that is to say the issuing of the starting request and the actual starting of the internal combustion engine, is kept as short as possible.
Specifically in diesel common rail systems it is extremely important to achieve a very rapid pressure build-up up to the injection release pressure when using a stop/start strategy.
In order to implement a stop/start functionality, different concepts can be employed depending on the components used in the high-pressure accumulator fuel injection system and, in particular, depending on the design of the high-pressure pump and of the injectors. Given a sufficiently large swept volume of the high-pressure pump, it is possible to build up within a defined time a pressure in the pressure accumulator (rail) which is larger than the injection release pressure (for example 80 bar). If low-leakage or leakage-free injectors are used in the high-pressure accumulator fuel injection system, use may also be made of what is referred to as a pressure holding function, in which the pressure in the pressure accumulator is held in the system for as long as possible.
If low-leakage or leakage-free injectors are used, depending on the pressure reduction strategy in the system a high-pressure control valve, referred to below for the sake of simplification as a pressure control valve (PCV), is necessary to achieve the pressure reduction times required by the automobile manufacturers.
If such a pressure control valve is embodied as a “normally open” design (when the electrical voltage is switched off a free through-flow of the fuel is possible), said valve has a defined holding pressure (for example 10-70 bar). If a fuel pressure in the system is held over the entire engine stop phase (for example 60-90 seconds in duration) within a stop/start cycle which is above the corresponding holding pressure, specifically at least the injection release pressure of the injectors which are used, the pressure control valve has to be supplied with electric current in order to prevent fuel from being removed from the pressure accumulator through the pressure control valve.
As a result of the necessary application of electric current, shortened below to energizing or energization, during the stop phase, electrical power is consumed. Since the internal combustion engine is deactivated in this phase and the generator (alternator) therefore cannot charge the vehicle accumulator, this has an adverse effect on the energy balance.
Hitherto, the requirements made of a stop/start functionality may be implemented by means of the most rapid possible pressure build-up at the start of the internal combustion engine.
If a pressure control valve with a normally closed function is used in high-pressure accumulator fuel injection systems with a pressure holding functionality, no energization is necessary during the stop phase. However, the use of such pressure control valves entails relatively high costs.
Furthermore, energizing a normally open pressure control valve during the stop phase by increasing the holding pressure can be reduced or avoided. However this increase is in conflict with the requirement for the lowest possible rail pressures during the idling of the internal combustion engine in order to reduce the noise level.